The use of image transfer members in electrostatic imaging is well known. Typically, an intermediate transfer surface is used to transfer a toner image from an imaging surface to a final substrate. This transfer is typically aided by heat and pressure.
Various types of intermediate transfer members are known and are described, for example in U.S. Pat. Nos. 3,862,848, 4,684,238, 4,690,539 and 4,531,825, the disclosures of all of which are incorporated herein by reference.
Belt-type intermediate transfer members for use in electrophotography are known in the art and are described, inter alia, in U.S. Pat. Nos. 3,893,761, 4,684,238 and 4,690,539, the disclosures of all of which are incorporated herein by reference.
The use of intermediate transfer members and members including transfer blankets for offset ink printing is also well known. Such blankets have characteristics which are suitable for ink transfer but are generally not usable, per se, for liquid toner imaging.
Desirably, the transfer of the toner image from the intermediate transfer surface onto the final substrate is complete. However, it is appreciated that some residual toner may remain on the surface after each transfer. The residual toner typically comprises deformed toner particles, some of which may be at least partially fused to other particles, which may adhere to the transfer surface and may accumulate to substantial amounts after many imaging cycles. This accumulation of the residual toner particles results in non-homogeneous and/or unclean transfer of the toner images onto the final substrate.
Cleaning of intermediate transfer members is known in the art. To enable continuous cleaning while avoiding erasure of the image being transferred, the cleaning station in other prior art devices is located downstream of the site at which the image is transferred onto the final substrate, prior to the transfer of a subsequent image to the intermediate transfer member.
In other known systems, the intermediate transfer member is periodically cleaned by printing a series of toner patterns, hereinafter referred to as "non-images", onto the final substrate. Printing of the non-images is based on applying a substantially continuous layer of fresh toner onto the intermediate transfer member and transferring the layer of liquid toner onto a final substrate, whereby the deformed residual toner particles adhere to the fresh toner and are thus removed from the intermediate transfer member. A substantially continuous or continuous layer of toner on the intermediate transfer member is typically obtained by developing a substantially continuous non-image on the imaging surface and transferring the developed non-image onto the intermediate transfer member.
To economize on the use of liquid toner, the substantially continuous non-image can be formed of a plurality of screen images each of which covers only a predetermined portion of the surface area of the intermediate transfer member. A sequence of such screen images, each preferably using a different color toner, provides complete coverage of the intermediate transfer member and collects substantially all the residual toner of all the color toners. It is appreciated that different color toners have different physical properties and, therefore, some color toners are more effective, e.g. more adhesive, than others in removing residual toner particles.
Cleaning of the intermediate transfer member by printing on the final substrate, as described above, generally requires at least eight imaging cycles for each cleaning session. The final substrate bearing the printed non-images which are formed during the cleaning session cannot be re-used and is, thus, discarded after the cleaning session, increasing maintenance costs. When the imaging system is designed for printing on a continuous final substrate as described, for example, in PCT publications WO 96/01442 and 96/31809, each cleaning session introduces a series of undesired non-images between consecutive images, interrupting the sequence of images formed on the final substrate.